1. Field of the Invention
The present invention relates to photoelectric conversion device using amorphous silicon and to a solid-state image sensing device comprising the same. More particularly, it relates to a photoelectric conversion device having a photoelectric multiplying function.
2. Description of Related Art
In recent years, attention has been focused on photoelectric conversion devices using amorphous silicon. Amorphous silicon has excellent affinity with monocrystalline silicon which is in the mainstream of current IC technology and thermally stable. Amorphous silicon is also advantageous in that it can be formed into a film at a low temperature, into a film occupying a large area, and into diverse structures. An avalanche multiplication photodiode (APD) of reach-through type using the amorphous silicon has been proposed as a high-sensitivity photoelectric conversion device.
The reach-through APD has a structure in which a carrier generation layer for absorbing light and generating carriers through optical excitation and a carrier multiplication layer for multiplying the carriers generated in the carrier generation layer with the application of high electric field are formed separately. There has also been proposed a step-back structure which provides the carrier multiplication layer with a plurality of heterojunction portions to produce a level difference in band gap in each of the junction portions and causes one multiplication after another with the level differences in band gap.
However, since the carrier generation layer and the carrier multiplication layer exist separately in the reach-through structure, it assumes a complicated structure consisting of a large number of layers so that interfaces between the layers are increased. Since the interfaces are likely to suffer defects, the number of defects produced at the interfaces is larger as the number of interfaces is larger. In a photoelectric conversion device using an amorphous material, an interface defect may cause a dark current. This is because carriers are generated through an interface state density produced by the interface defect upon the application of an extremely high electric field, which should be applied to the multiplication layer in order to effect a phenomenon of avalanche multiplication. In a device with the step-back structure having a plurality of level differences in band gap in the carrier multiplication layer, the number of interfaces is particularly increased and the dark current resulting from the interface defect is increased notably. As a consequence, the reach-through APD using an amorphous material has not been used practically.